Profile extrusion of thermoplastics is often difficult due to the poor melt integrity of the thermoplastics. This is particularly a problem in extruded profiles of internally lubricated thermoplastic composites which exhibit poor surface appearance.
In the past several years, Petrarch Systems Inc. of Bristol, Pa. has developed a new class of melt processable thermoplastics which contain silicone interpenetrating polymer networks (IPN's). The compositions are sometimes referred to as semi-interpenetrating polymer networks (pseudo- or semi-IPN's) since only the silicone component is vulcanized or cross-linked in the thermoplastic matrix. The interpenetrating polymer network is formed by vulcanizing a hydride functional silicone group and a polymer containing at least one unsaturated group, preferably a vinyl group, during normal thermoplastic melt processing. See U.S. Pat. No. 4,500,688, issued Feb. 19, 1987, and Barry C. Arkles, U.S. Pat. No. 4,714,739, issued Dec. 22, 1987.
The pseudo- or semi-IPN's have the advantages of exhibiting surface and di-electric properties which approach those of silicones and mechanical properties which approach those of the thermoplastic matrix. In many cases, properties such as wear, lubricity and heat and abrasion resistance of the thermoplastics are improved. While this technology was originally developed in polyurethane systems, it has been extended to other thermoplastic elastomers and engineering thermoplastics. For example, U.S. Pat. No. 4,714,739, assigned to Petrarch Systems Inc. discloses that polyamide (nylon) and polyester engineering thermoplastics have been formulated into materials which appear to be particularly well suited for gear and bearing applications, with the heat distortion temperature of nylon 6,6 being increased by the incorporation of 5 wt % silicone IPN.
However, the major uses of silicone IPN's have still been in the modification of lower modulus thermoplastics, particularly elastomers. To applicants' knowledge, the silicone IPN's have not been used or recognized to improve melt integrity characteristics of high modulus thermoplastic materials, i.e., thermoplastic materials which in the unfilled state have a flexural modulus greater than 90,000 psi, as measured by ASTM D790.